Practical Rocketry

The first rockets ever built, the fire-arrows of the Chinese, were not very
reliable. Many just exploded on launching. Others flew on erratic courses
and landed in the wrong place. Being a rocketeer in the days of the fire-arrows
must have been an exciting, but also a highly dangerous activity.

Today, rockets are much more reliable. They fly on precise courses and
are capable of going fast enough to escape the gravitational pull of Earth.
Modern rockets are also more efficient today because we have an understanding
of the scientific principles behind rocketry. Our understanding has led
us to develop a wide variety of advanced rocket hardware and devise new
propellants that can be used for longer trips and more powerful takeoffs.

Rocket Engines and Their Propellants

Most rockets today operate with either solid or liquid propellants. The
word propellant does not mean simply fuel, as you might think; it
means both fuel and oxidizer. The fuel is the chemical the
rocket burns but, for burning to take place, an oxidizer (oxygen) must be
present. Jet engines draw oxygen into their engines from the surrounding
air. Rockets do not have the luxury that jet planes have; they must carry
oxygen with them into space, where there is no air.

Solid rocket propellants, which are dry to the touch, contain both the
fuel and oxidizer combined together in the chemical itself. Usually the
fuel is a mixture of hydrogen compounds and carbon and the oxidizer is
made up of oxygen compounds. Liquid propellants, which are often gases
that have been chilled until they turn into liquids, are kept in separate
containers, one for the fuel and the other for the oxidizer. Just before
firing, the fuel and oxidizer are mixed together in the engine.

Solid Propellant Rocket

A solid-propellant rocket has the simplest form of engine. It has
a nozzle, a case, insulation, propellant, and an igniter.
The case of the engine is usually a relatively thin metal that is lined
with insulation to keep the propellant from burning through. The propellant
itself is packed inside the insulation layer.

Many solid-propellant rocket engines feature a hollow core that runs
through the propellant. Rockets that do not have the hollow core must
be ignited at the lower end of the propellants and burning proceeds gradually
from one end of the rocket to the other. In all cases, only the surface
of the propellant burns. However, to get higher thrust, the hollow core
is used. This increases the surface of the propellants available for burning.
The propellants burn from the inside out at a much higher rate, sending
mass out the nozzle at a higher rate and speed. This results in greater
thrust. Some propellant cores are star shaped to increase the burning
surface even more.

To ignite solid propellants, many kinds of igniters can be used. Fire-arrows
were ignited by fuses, but sometimes these ignited too quickly and burned
the rocketeer. A far safer and more reliable form of ignition used today
is one that employs electricity. An electric current, coming through wires
from some distance away, heats up a special wire inside the rocket. The
wire raises the temperature of the propellant it is in contact with to
the combustion point.

Other igniters are more advanced than the hot wire device. Some are
encased in a chemical that ignites first, which then ignites the propellants.
Still other igniters, especially those for large rockets, are rocket engines
themselves. The small engine inside the hollow core blasts a stream of
flames and hot gas down from the top of the core and ignites the entire
surface area of the propellants in a fraction of a second.

The nozzle in a solid-propellant engine is an opening at the back of
the rocket that permits the hot expanding gases to escape. The narrow
part of the nozzle is the throat. Just beyond the throat is the
exit cone.

The purpose of the nozzle is to increase the acceleration of the gases
as they leave the rocket and thereby maximize the thrust. It does this
by cutting down the opening through which the gases can escape. To see
how this works, you can experiment with a garden hose that has a spray
nozzle attachment. This kind of nozzle does not have an exit cone, but
that does not matter in the experiment. The important point about the
nozzle is that the size of the opening can be varied.

Start with the opening at its widest point. Watch how far the water
squirts and feel the thrust produced by the departing water. Now reduce
the diameter of the opening, and again note the distance the water squirts
and feel the thrust. Rocket nozzles work the same way.

As with the inside of the rocket case, insulation is needed to protect
the nozzle from the hot gases. The usual insulation is one that gradually
erodes as the gas passes through. Small pieces of the insulation get very
hot and break away from the nozzle. As they are blown away, heat is carried
away with them.

Liquid Propellant Rocket

The other main kind of rocket engine is one that uses liquid propellants,
which may be either pumped or fed into the engine by pressure. This is a
much more complicated engine, as is evidenced by the fact that solid rocket
engines were used for at least seven hundred years before the first successful
liquid engine was tested. Liquid propellants have separate storage tanks--one
for the fuel and one for the oxidizer. They also have a combustion chamber,
and a nozzle.

The fuel of a liquid-propellant rocket is usually
kerosene or liquid hydrogen; the oxidizer is usually liquid oxygen. They
are combined inside a cavity called the combustion chamber . Here the
propellants burn and build up high temperatures and pressures, and the
expanding gas escapes through the nozzle at the lower end. To get the
most power from the propellants, they must be mixed as completely as possible.
Small injectors (nozzles) on the roof of the chamber spray and
mix the propellants at the same time. Because the chamber operates under
high pressures, the propellants need to be forced inside. Modern liquid
rockets use powerful, lightweight turbine pumps to take care of this job.

With any rocket, and especially with liquid-propellant rockets, weight
is an important factor. In general, the heavier the rocket, the more the
thrust needed to get it off the ground. Because of the pumps and fuel
lines, liquid engines are much heavier than solid engines.

One especially good method of reducing the weight of liquid engines
is to make the exit cone of the nozzle out of very lightweight metals.
However, the extremely hot, fast-moving gases that pass through the cone
would quickly melt thin metal. Therefore, a cooling system is needed.
A highly effective though complex cooling system that is used with some
liquid engines takes advantage of the low temperature of liquid hydrogen.
Hydrogen becomes a liquid when it is chilled to -253° C. Before injecting
the hydrogen into the combustion chamber, it is first circulated through
small tubes that lace the walls of the exit cone. In a cutaway view, the
exit cone wall looks like the edge of corrugated cardboard. The hydrogen
in the tubes absorbs the excess heat entering the cone walls and prevents
it from melting the walls away. It also makes the hydrogen more energetic
because of the heat it picks up. We call this kind of cooling system regenerative
cooling.

Engine Thrust Control

Controlling the thrust of an engine is very important to launching payloads
(cargoes) into orbit. Thrusting for too short or too long of a period of
time will cause a satellite to be placed in the wrong orbit. This could
cause it to go too far into space to be useful or make the satellite fall
back to Earth. Thrusting in the wrong direction or at the wrong time will
also result in a similar situation.

A computer in the rocket's guidance system determines when that thrust
is needed and turns the engine on or off appropriately. Liquid engines
do this by simply starting or stopping the flow of propellants into the
combustion chamber. On more complicated flights, such as going to the
Moon, the engines must be started and stopped several times.

Some liquid-propellant engines control the amount of engine thrust by
varying the amount of propellant that enters the combustion chamber. Typically
the engine thrust varies for controlling the acceleration experienced
by astronauts or to limit the aerodynamic forces on a vehicle.

Solid-propellant rockets are not as easy to control as liquid rockets.
Once started, the propellants burn until they are gone. They are very
difficult to stop or slow down part way into the burn. Sometimes fire
extinguishers are built into the engine to stop the rocket in flight.
But using them is a tricky procedure and does not always work. Some solid-fuel
engines have hatches on their sides that can be cut loose by remote control
to release the chamber pressure and terminate thrust.

The burn rate of solid propellants is carefully planned in advance.
The hollow core running the length of the propellants can be made into
a star shape. At first, there is a very large surface available for burning,
but as the points of the star burn away, the surface area is reduced.
For a time, less of the propellant burns, and this reduces thrust. The
Space Shuttle uses this technique to reduce vibrations early in
its flight into orbit.

NOTE: Although most rockets used by governments and research organizations
are very reliable, there is still great danger associated with the building
and firing of rocket engines. Individuals interested in rocketry should
never attempt to build their own engines. Even the simplest-looking
rocket engines are very complex. Case-wall bursting strength, propellant
packing density, nozzle design, and propellant chemistry are all design
problems beyond the scope of most amateurs. Many home-built rocket engines
have exploded in the faces of their builders with tragic consequences.

Stability and Control Systems

Building an efficient rocket engine is only part of the problem in producing
a successful rocket. The rocket must also be stable in flight. A stable
rocket is one that flies in a smooth, uniform direction. An unstable rocket
flies along an erratic path, sometimes tumbling or changing direction. Unstable
rockets are dangerous because it is not possible to predict where they will
go. They may even turn upside down and suddenly head back directly to the
launch pad.

Making a rocket stable requires some form of control system. Controls
can be either active or passive. The difference between these and how
they work will be explained later. It is first important to understand
what makes a rocket stable or unstable.

All matter, regardless of size, mass, or shape, has a point inside called
the center of mass (CM). The center of mass is the exact spot where
all of the mass of that object is perfectly balanced. You can easily find
the center of mass of an object such as a ruler by balancing the object
on your finger. If the material used to make the ruler is of uniform thickness
and density, the center of mass should be at the halfway point between
one end of the stick and the other. If the ruler were made of wood, and
a heavy nail were driven into one of its ends, the center of mass would
no longer be in the middle. The balance point would then be nearer
the end with the nail.

The center of mass is important in rocket flight because it is around
this point that an unstable rocket tumbles. As a matter of fact, any object
in flight tends to tumble. Throw a stick, and it tumbles end over end.
Throw a ball, and it spins in flight. The act of spinning or tumbling
is a way of becoming stabilized in flight. A Frisbee will go where you
want it to only if you throw it with a deliberate spin. Try throwing a
Frisbee without spinning it. If you succeed, you will see that the Frisbee
flies in an erratic path and falls far short of its mark.

In flight, spinning or tumbling takes place around one or more of three
axes. They are called roll, pitch, and yaw. The point
where all three of these axes intersect is the center of mass. For rocket
flight, the pitch and yaw axes are the most important because any movement
in either of these two directions can cause the rocket to go off course.
The roll axis is the least important because movement along this axis
will not affect the flight path. In fact, a rolling motion will help stabilize
the rocket in the same way a properly passed football is stabilized by
rolling (spiraling) it in flight. Although a poorly passed football may
still fly to its mark even if it tumbles rather than rolls, a rocket will
not. The action-reaction energy of a football pass will be completely
expended by the thrower the moment the ball leaves the hand. With rockets,
thrust from the engine is still being produced while the rocket is in
flight. Unstable motions about the pitch and yaw axes will cause the rocket
to leave the planned course. To prevent this, a control system is needed
to prevent or at least minimize unstable motions.

In addition to center of mass, there is another important center inside
the rocket that affects its flight. This is the center of pressure
(CP). The center of pressure exists only when air is flowing past the
moving rocket. This flowing air, rubbing and pushing against the outer
surface of the rocket, can cause it to begin moving around one of its
three axes. Think for a moment of a weather vane. A weather vane is an
arrow-like stick that is mounted on a rooftop and used for telling wind
direction. The arrow is attached to a vertical rod
that acts as a pivot point. The arrow is balanced so that the center of
mass is right at the pivot point. When the wind blows, the arrow turns,
and the head of the arrow points into the oncoming wind. The tail of the
arrow points in the downwind direction.

The reason that the weather vane arrow points into the wind is that
the tail of the arrow has a much larger surface area than the arrowhead.
The flowing air imparts a greater force to the tail than the head, and
therefore the tail is pushed away. There is a point on the arrow where
the surface area is the same on one side as the other. This spot is called
the center of pressure. The center of pressure is not in the same place
as the center of mass. If it were, then neither end of the arrow would
be favored by the wind and the arrow would not point. The center of pressure
is between the center of mass and the tail end of the arrow. This means
that the tail end has more surface area than the head end.

It is extremely important that the center of pressure in a rocket be
located toward the tail and the center of mass be located toward the nose.
If they are in the same place or very near each other, then the rocket
will be unstable in flight. The rocket will then try to rotate about the
center of mass in the pitch and yaw axes, producing a dangerous situation.
With the center of pressure located in the right place, the rocket will
remain stable.

Control systems for rockets are intended to keep a rocket stable in
flight and to steer it. Small rockets usually require only a stabilizing
control system. Large rockets, such as the ones that launch satellites
into orbit, require a system that not only stabilizes the rocket, but
also enable it to change course while in flight.

Controls on rockets can either be active or passive. Passive controls
are fixed devices that keep rockets stabilized by their very presence
on the rocket's exterior. Active controls can be moved while the
rocket is in flight to stabilize and steer the craft.

The simplest of all passive controls is a stick. The Chinese fire-arrows
were simple rockets mounted on the ends of sticks. The stick kept the
center of pressure behind the center of mass. In spite of this, fire-arrows
were notoriously inaccurate. Before the center of pressure could take
effect, air had to be flowing past the rocket. While still on the ground
and immobile, the arrow might lurch and fire the wrong way.

Years later, the accuracy of fire-arrows was improved considerably by
mounting them in a trough aimed in the proper direction. The trough guided
the arrow in the right direction until it was moving fast enough to be
stable on its own.

As will be explained in the next section, the weight of the rocket is
a critical factor in performance and range. The fire-arrow stick added
too much dead weight to the rocket, and therefore limited its range considerably.

An important improvement in rocketry came with the
replacement of sticks by clusters of lightweight fins mounted around the
lower end near the nozzle. Fins could be made out of lightweight materials
and be streamlined in shape. They gave rockets a dart-like appearance.
The large surface area of the fins easily kept the center of pressure
behind the center of mass. Some experimenters even bent the lower tips
of the fins in a pinwheel fashion to promote rapid spinning in flight.
With these "spin fins," rockets become much more stable

in flight. But this design also produces more drag and limits the rocket's
range.

With the start of modern rocketry in the 20th century, new ways were
sought to improve rocket stability and at the same time reduce overall
rocket weight. The answer to this was the development of active controls.
Active control systems included vanes, movable fins, canards,
gimbaled nozzles, vernier rockets, fuel injection,
and attitude-control rockets. Tilting fins and canards are quite
similar to each other in appearance. The only real difference between
them is their location on the rockets. Canards are mounted on the front
end of the rocket while the tilting fins are at the rear. In flight, the
fins and canards tilt like rudders to deflect the air flow and cause the
rocket to change course. Motion sensors on the rocket detect unplanned
directional changes, and corrections can be made by slight tilting of
the fins and canards. The advantage of these two devices is size and weight.
They are smaller and lighter and produce less drag than the large fins.

Other active control systems can eliminate fins and canards altogether.
By tilting the angle at which the exhaust gas leaves the rocket engine,
course changes can be made in flight. Several techniques can be used for
changing exhaust direction.

Vanes are small finlike devices that are placed inside the exhaust of
the rocket engine. Tilting the vanes deflects the exhaust, and by action-reaction
the rocket responds by pointing the opposite way.

Another method for changing the exhaust direction is to gimbal the nozzle.
A gimbaled nozzle is one that is able to sway while exhaust gases are
passing through it. By tilting the engine nozzle in the proper direction,
the rocket responds by changing course.

Vernier rockets can also be used to change direction. These are small
rockets mounted on the outside of the large engine. When needed they fire,
producing the desired course change.

In space, only by spinning the rocket along the roll axis or by using
active controls involving the engine exhaust can the rocket be stabilized
or have its direction changed. Without air, fins and canards have nothing
to work upon. (Science fiction movies showing rockets in space with wings
and fins are long on fiction and short on science.) While coasting in
space, the most common kinds of active control used are attitude-control
rockets. Small clusters of engines are mounted all around the vehicle.
By firing the right combination of these small rockets, the vehicle can
be turned in any direction. As soon as they are aimed properly, the main
engines fire, sending the rocket off in the new direction.

Mass

Mass is another important factor affecting the performance of a rocket.
The mass of a rocket can make the difference between a successful flight
and just wallowing around on the launch pad. As a basic principle of rocket
flight, it can be said that for a rocket to leave the ground, the engine
must produce a thrust that is greater than the total mass of the vehicle.
It is obvious that a rocket with a lot of unnecessary mass will not be as
efficient as one that is trimmed to just the bare essentials.

For an ideal rocket, the total mass of the vehicle should be distributed
following this general formula:

Of the total mass, 91 percent should be propellants; 3 percent
should be tanks, engines, fins, etc.; and 6 percent can be the payload.

Payloads may be satellites, astronauts, or spacecraft that will travel to
other planets or moons.

In determining the effectiveness of a rocket design, rocketeers speak
in terms of mass fraction (MF). The mass of the propellants of the rocket
divided by the total mass of the rocket gives mass fraction:

MF =

mass of propellants
total mass

The mass fraction of the ideal rocket given above is 0.91. From the
mass fraction formula one might think that an MF of 1.0 is perfect, but
then the entire rocket would be nothing more than a lump of propellants
that would simply ignite into a fireball. The larger the MF number, the
less payload the rocket can carry; the smaller the MF number, the less
its range becomes. An MF number of 0.91 is a good balance between payload-carrying
capability and range. The Space Shuttle has an MF of approximately 0.82.
The MF varies between the different orbiters in the Space Shuttle fleet
and with the different payload weights of each mission.

Large rockets, able to carry a spacecraft into space, have serious weight
problems. To reach space and proper orbital velocities, a great deal of
propellant is needed; therefore, the tanks, engines, and associated hardware
become larger. Up to a point, bigger rockets can carry more payload than
smaller rockets, but when they become too large their structures weigh
them down too much, and the mass fraction is reduced to an impossible
number.

A solution to the problem of giant rockets weighing too much can be
credited to the 16th-century fireworks maker Johann Schmidlap. Schmidlap
attached small rockets to the top of big ones. When the large rocket was
exhausted, the rocket casing was dropped behind and the remaining rocket
fired. Much higher altitudes were achieved by this method. (The Space
Shuttle follows the step rocket principle by dropping off its solid rocket
boosters and external tank when they are exhausted of propellants.)

The rockets used by Schmidlap were called step rockets. Today this technique
of building a rocket is called staging. Thanks to staging, it has
become possible not only to reach outer space but the Moon and other planets
too.